SUBSCRIBER TRUNK DIALLING AND THE TRANSIT NETWORK


By 1980 subscriber trunk dialling had grown from nothing to become universal. The 1957White Paper proposing simplified call charging and automation heralded “the most sweeping and radical reform since the Post Office took over the telephone service from the  National Telephone Company  in 1912.The system was inaugurated at Bristol on 5th  December 1958 by HM the Queen and was completed 21 years later in 1979 although  within 10 years nearly 80% of all subscribers had access to STD.

The fundamental tariff changes enabled exchanges to be grouped into about 600 charging groups, reducing the charge list for any given exchange from about 6000. A new trunk switching and transmission plan met the demands of system whereby control of trunk calls passed from the operator an electromechanical or electronic device which would route the call and raise the appropriate charge. This was at first known as GRACE (Group Routing and Charging Equipment) but was in fact a register translator. The original installation at Bristol was electronic, employing cold cathode discharge tubes while magnetic drum register translators were used in large director area installations.

At that time of course the actual switching was by Strowger type selectors, but commencing in 1973 at Dover the group switching centres began to be modernised and expanded with crossbar switches which were compatible with electronic Stored Program Control. The first phase of STD provided for the development of a system that  called for a great majority of calls using the existing network as about 95% of calls are carried on only about 40% of the routings. To maintain a satisfactory transmission standard no more than two GSC/GSC links to tandem is permissible, further there is a limit to the allowable post-dialling delay before the caller receives ringing or other tone. To meet these requirements, a transit network using fast signalling and fast switching was introduced in 1969 using crossbar switching. It was the provision of this transit network which allowed the completion of the task of the second phase providing STD for all telephones to be completed. [Stan Roberts]


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